A Simple and Efficient Protocol for the Catalytic Insertion Polymerization of Functional Norbornenes

Summary

We describe the catalytic insertion polymerization of 5-norbornene-2-carboxylic acid and 5-vinyl-2-norbornene to form functional polymers with a very high glass transition temperature.

Abstract

Norbornene can be polymerized by a variety of mechanisms, including insertion polymerization whereby the double bond is polymerized and the bicyclic nature of the monomer is conserved. The resulting polymer, polynorbornene, has a very high glass transition temperature, T_g, and interesting optical and electrical properties. However, the polymerization of functional norbornenes by this mechanism is complicated by the fact that the endo substituted norbornene monomer has, in general, a very low reactivity. Furthermore, the separation of the endo substituted monomer from the exo monomer is a tedious task. Here, we present a simple protocol for the polymerization of substituted norbornenes (endo:exo ca. 80:20) bearing either a carboxylic acid or a pendant double bond. The process does not require that both isomers be separated, and proceeds with low catalyst loadings (0.01 to 0.02 mol%). The polymer bearing pendant double bonds can be further transformed in high yield, to afford a polymer bearing pendant epoxy groups. These simple procedures can be applied to prepare polynorbornenes with a variety of functional groups, such as esters, alcohols, imides, double bonds, carboxylic acids, bromo-alkyls, aldehydes and anhydrides.

Introduction

Norbornene, NBE, the Diels-Alder adduct of ethylene and cyclopentadiene (obtained by "cracking" of dicyclopentadiene (DCPD)), is readily polymerized using either free-radical polymerization,¹ cationic polymerization,² ring-opening metathesis polymerization³ and catalytic insertion polymerization.^4,5,6,7 Unlike the other mechanisms, the catalytic insertion polymerization leads to the formation of a very high glass-transition temperature (T_g) polymer whereby the bicyclic backbone of NBE is conserved. A variety of catalysts such as metallocene catalysts and late transition metal catalysts can be used to promote the polymerization of NBE.^4,5,6,7 However, due to its low solubility and due to difficulties associated with the processing of a very high T_g polymer, the PNBE homopolymer has, to our knowledge, never found any use.

Functional polynorbornenes (PNBEs) have been the object of considerable scrutiny for the last 20 years, because they combine the high T_g imparted by the bicyclic rigid repeat unit as well as desirable properties endowed by their functionalities.^8,9,10 NBE monomers are obtained from rather simple and inexpensive feedstocks, using a one-step Diels-Alder reaction between cyclopentadiene and a functionalized dienophile. However, the Diels-Alder reaction leads to two stereoisomers, endo and exo, which have very different reactivities.^11,12 In fact, the endo stereoisomer is less reactive than exo form and deactivates the catalyst.^11,12 Thus, in the past, the preparation of functional polynorbornenes usually required the separation of the endo and exo stereoisomers, and only the exo stereoisomer was used. Such a separation procedure was time-consuming, and led to the accumulation of unreacted endo stereoisomers as undesirable waste.

Recently we have shown that the polymerization of functionalized NBEs containing both stereoisomers is in fact feasible.¹³ We have thus been able to prepare a variety of substituted PNBEs, containing functional groups such as esters, anhydrides, aldehydes, imides, alcohols and double bonds. Due to their high T_g and functionality, these polymers show desirable properties. We describe here two methods to prepare functional polymers. The first one leads to the synthesis of the water soluble polymer poly(5-norbornene-2-carboxylic acid), PNBE(CO₂H), using a cationic Pd catalyst (Figure 1).^13,14 The same polymerization method can be used to prepare functional PNBEs with various pendant functionalities, such as esters, alcohols, imides, bromo-alkyls, aldehydes and anhydrides. In our hands, this cationic Pd catalyst cannot be used for NBEs containing pendant double bonds such as 5-vinyl-2-norbornene. In this case, a partial insertion of the pendant double bond during the polymerization leads to the formation of a cross-linked material. Therefore, we present here a second method dedicated to the formation of poly(5-vinyl-2-norbornene), PNBE(vinyl), using Pd₂(dba)₃:AgSbF₆:PPh₃ as an in situ catalyst.¹⁴ The pendant vinyl groups of the polymer are then further epoxidized, to lead to the formation of PNBE(epoxy) (Figure 1). Both PNBE(CO₂H) and PNBE(epoxy) have been found to lead to the formation of thermoset resins with a T_g as high as 350 °C.¹⁴ Thus, the simple method described here allows one to efficiently prepare polymers with a very high T_g and having a variety of functional groups, which can be used for numerous applications.

Figure 1: Functional PNBEs prepared by Pd catalyzed polymerization. (A) preparation of PNBE(CO₂H), (B) preparation of PNBE(vinyl) and PNBE(epoxy). The dashed bond indicates a mixture of endo and exo isomers. Please click here to view a larger version of this figure.

Protocol

1. Preparation of Poly(5-norbornene-2-carboxylic acid), PNBE(CO2H) Preparation of the monomer NBE(CO2H) Weigh out acrylic acid (AA) (327 g, 4.5 mol, 2 eq.) and hydroquinone (4.9 g, 4.5 x 10-2 mol, 0.02 eq.) and add them to a 2 L round-bottom flask equipped with a condenser and a magnetic stir bar. Heat the flask at 150 °C using a silicone oil bath. Once reflux is settled, add DCPD (300 g, 2.3 mol, 1 eq.) in a single portion, and the…

Representative Results

The NBE monomers are prepared by simple Diels-Alder reaction of DCPD and a suitable dienophile, for example acrylic acid (AA). Normally, DCPD is cracked to yield cyclopentadiene (CPD) before reaction.17 Freshly cracked CPD is then engaged in the Diels-Alder reaction. However, in this protocol, both cracking and Diels-Alder steps are performed concomitantly, in a one-pot reaction. Thus, as soon as CPD is formed, it reacts with AA to yield 5-norbornene-2-carboxylic a…

Discussion

The method proposed here is simple, and readily amenable to scale-up. All chemicals could be used as received without any purification. Note that performing the reaction at a lower scale (e.g. scales ≤1 g) usually yields lower yields due to an unavoidable loss of material during the handling and the collection.

The catalysts are formed in situ upon the reaction of commercial Pd compounds with cationizing agents. In our hands, the yield of the reaction as well as the cha…

Materials

acrylic acid	Sigma-Aldrich	147230
hydroquinone	Sigma-Aldrich	H9003
dicyclopendadiene	Sigma-Aldrich	454338
palladium allyl dichloride dimer	Sigma-Aldrich	222380
silver hexfluoro antimonate	Sigma-Aldrich	227730
liquid nitrogen	Local Facility	NA
ethyl acetate	Fischer Scientific	E14520
5-vinyl-2-norbornene	Sigma-Aldrich	148679
toluene	Fischer Scientific	T290-4
palladium dba	Sigma-Aldrich	227994
triphenyl phosphine	Sigma-Aldrich	93090
silica gel 40-63 microns	Silicycle	Siliaflash
methanol	Fischer Scientific	BPA412-20
dichloromethane	EMD Millipore	DX08311
formic acid	Sigma-Aldrich	F0507
acetic acid	Sigma-Aldrich	320099
hydrogen peroxide solution	Sigma-Aldrich	216763
acetone	Fischer Scientific	A18-200